"nud" means "Neighbour Unreachability Detection." The first line of output tells you the
IP, interface, and hardware (MAC) address, and that the host is up and reachable. On the
second line "nud stale" means the host has not been contacted recently and the entry is
ready to time out. Typically an entry in the cache has a lifetime of about 15 minutes. To
refresh the ARP table entry simply ping the host.

Building A Router
To do actual routing requires that certain features are enabled in the kernel. All you
need for basic routing is

CONFIG_NETLINK_DEV=m

This sample from /boot/config-2.6.8 file shows a complete routing feature set
enabled. This shows every routing-related option available: